Use of low molecular-weight or polymeric organic compounds which are present in the columnar-helical phase and have liquid-crystalline properties

ABSTRACT

The use of low molecular-weight or polymeric organic compounds which are present in the columnar-helical phase and have liquid-crystalline properties, as photoconductors or in electronic components, corresponding photoconductive layers, an electrophotographic recording material and a method for enhancing the photoconductivity.

The invention relates to the use of low molecular-weight or polymericorganic compounds which are present in the columnar-helical phase andhave liquid-crystalline properties, as photoconductors or in electroniccomponents, corresponding photoconductive layers, an electrophotographicrecording material and a method for enhancing the photoconductivity.

Photoconductive polymers are an interesting class of materials and areused industrially on a large scale in copiers, laser printers and offsetprinting plates.

A number of attempts to improve the charge transfer characteristics ofliquid-crystalline materials by orientation in the liquid-crystallinestate are known. There are three approaches to this objective.

Firstly, low molecular-weight liquid-crystallines which form nematicphases at room temperature are doped with carbazole, the absorptioncapacity of the liquid-crystalline matrix being very limited, however.Even at very low concentrations (a few wt. %) the carbazole begins tocrystallize. Accordingly, only a small photocurrent of thecarbazole-doped liquid-crystalline samples is produced (compare L. L.Chapoy, D. K. Munck, K. H. Rasmussen, E. Juul-Diekmann, R. K. Sethi, D.Biddle, in Molecular Crystals, liquid-crystallines, Vol. 105, p. 353 etseq. (1984)).

The second approach is illustrated in EP-A-0 254 060. EP-A-0 254 060discloses photoconductive films having a thickness of less than 20 μm,which are produced from concentrated lyophase solutions of a polymerwhich has a repeating unit according to the general formula I, ##STR1##R being an alkylene radical having from 1 to 20 carbon atoms and P beinga photoconductive group.

The third approach is described in EP-A-0 527 376. This describes lowmolecular-weight and polymeric organic photoconductors having generallydiskotic liquid-crystalline properties with enhanced photoconductivity.A typical representative is hexaalkyloxytriphenylene, which has carriercharge mobilities of almost 10⁻³ cm² /Vs (compare D. Adam, F. Closs, T.Frey, D. Funhoff, D. Haarer, H. Ringsdorf, R. Schuhmacher, K.Siemensmeyer, Phys. Rev. Lett., 1993, 70, 457).

It is an object of the present invention to provide organic compoundshaving liquid-crystalline properties, which in the liquid-crystallinestate have an even higher photoconductivity or charge carrier mobilityand which are suitable for use as photoconductors or in electroniccomponents.

We have found that this object is achieved by the use of lowmolecular-weight or polymeric organic compounds which are present in thecolumnar-helical phase and have liquid-crystalline properties, asphotoconductors or in electronic components.

Another object of the invention is a photoconductive layer whichcomprises a low molecular-weight or polymeric organic compound which ispresent in the columnar-helical phase and has liquid-crystallineproperties.

A further object of the invention is an electrophotographic recordingmaterial which comprises an electroconductive base and a photoconductivelayer of the abovementioned type.

The organic compounds used according to the invention, which haveliquid-crystalline properties, generally have a photoconductivity ofgreater than 10⁻² cm² /Vs.

Low molecular-weight organic compounds having a columnar-helical phasewhich are suitable according to the invention include, for example,hexahexylthiotriphenylene and its mixtures with otherhexaalkylthio-substituted derivatives, but also otherwise substitutedtriphenylenes, phthalocyanines, hexasubstituted benzenes, truxenes andhexa- or octasubstituted dibenzopyrenes, which have a columnar-helicalphase.

Among these organic compounds triphenylene derivatives are preferred,particular preference in turn being given to triphenylene derivativeswhich contain thioalkyl substituents. A particularly suitable lowmolecular-weight organic compound which has a high charge carriermobility and photoconductivity in a columnar-helical phase is2,3,6,7,10,11-hexahexylthiotriphenylene.

The synthesis of 2,3,6,7,10,11-hexahexylthiotriphenylene and itscolumnar-helical phase are described in Molecular Crystals,liquid-crystallines 1991, Vol. 198, pp. 273 to 284.

The invention further relates to triphenylene derivatives of the generalformula I, as novel compounds, ##STR2## where the radicals

R are C₄ - to C₈ -n-alkyl groups in random distribution.

Among the compounds of formula I, those having C₅ - and C₆ -n-alkylradicals are preferred. The random distribution preferably involves from70 to 80% of n-hexyl and from 20 to 30% of n-pentyl.

To prepare the compounds containing the randomly distributed alkylradicals, hexabromotriphenylene can be reacted with suitable mixtures ofthe C₄ - to C₈ -n-alkylthiols, preferably in the form of the alkalimetal salts. Details of the reaction can be gathered from the example.

Polymeric organic compounds suitable according to the invention arethose which contain photoconductive groups, either in the polymer chainor bound to a polymer chain via a flexible spacer, and have acolumnar-helical phase.

According to the invention, low molecular-weight compounds can beemployed which contain one or more polymerizable groups which arecrosslinked in the columnar-helical phase.

Equally possible is the formation of charge-transfer complexes togenerate charge carriers. According to the invention, the lowmolecular-weight or polymeric organic compounds havingliquid-crystalline properties are able to act as electron donors or aselectron acceptors in the charge-transfer complexes. As a rule, the saidlow molecular-weight or polymeric organic compounds havingliquid-crystalline properties act as electron donors.

The low molecular-weight or polymeric organic compounds used accordingto the invention are known per se or can be prepared according tocustomary methods (compare B. Kohne, W. Poules and K. Praefcke,Chemiker-Zeitung, 108 (1984), No. 3, p. 113;

EP-A-0 527 376; U.S. Pat. No. 4,865,762).

The uses according to the invention as a rule involve thinphotoconductive layers, in some cases providing the option of separatingthe charge transfer from the charge generation in the sense of atwo-layer arrangement, as used in electrophotography. In thisarrangement the photoconductor is situated in the photoconductive chargetransfer layer which is adjoined by a conventional and known sensitizerlayer which generates known charge carriers. In this case charging isusually effected via a high-voltage corona.

To enhance the photosensitivity of the photoconductive layers,sensitizers, i.e. compounds which generate charge carriers but do notdestroy the columnar-helical phase, can be added. Compounds of this typeinclude e.g. the perylenetetracarboxylic acid derivatives disclosed byDE-A 22 37 539 and DE-A 31 10 955. Particular preference is given to theaddition of liquid-crystalline compounds as charge carrier generators.

The layers according to the invention can be produced on a substratesurface by application of a melt or, in a customary and known manner,e.g. by blade-coating a substrate surface with a solution of thecompounds. In so doing, the solution may be admixed with variousadjuvants, e.g. to improve the leveling characteristics. Also suitableare spin-coating and Langmuir-Blodgett techniques.

Solvents used are, for example, tetrahydrofuran or dichloromethane.

These photoconductive layers generally have a layer thickness of between2 and 100, preferably of between 4 and 50 and particularly preferablybetween 4 and 30 μm.

The photoconductors or the photoconductive layers may be sandwichedbetween conductively coated, transparent substrates, for which glassplates or plates of optically transparent plastics (for examplepoly(methyl methacrylate), polycarbonate etc.) are used. The conductivecoating of the substrate may comprise electroconductive polymers,semiconductors or metals, although the thickness of said coating shouldbe chosen so as not unduly to impair the optical transmittance.Particularly advantageous coatings comprise ITO (indium tin oxide).

To generate a photocurrent, a DC voltage of between 5 and 50 V is thengenerally applied to the electroconductively coated plates.

The use, according to the invention, as photoconductors involvesutilization of the enhanced charge carrier mobility andphotoconductivity, whereas the use in electronic components admittedlyalso involves the utilization of the photoconductivity, but above all ofthe enhanced charge carrier mobility in the absence of light.

The photoconductors and photoconductive films according to the inventioncan be used in electrophotography, in laser printers, in offset printingor alternatively in microelectronics for photosensitive switches.

In addition, the novel photoconductive layers can be employed in allthose sectors in which the enhanced photoconductivity brought about byan ordered molecular arrangement can be utilized.

According to the invention, the low molecular-weight or polymericorganic compounds which are present in the helical phase and haveliquid-crystalline properties can also be used in electronic components,the term electronic components for the purposes of the inventionpreferentially being intended to refer to those in which the enhancedcharge carrier mobility rather than the enhanced photoconductivity isutilized.

The use in electronic components also includes the application indisplays.

The application in displays provides the option, according to theinvention, of fabricating LEDs (light-emitting diodes), the phenomenonof electroluminescence being utilized. The organic substances which areused according to the invention in the columnar-helical phase areparticularly suitable owing to their high chemical stability withrespect to electrical fields applied and high temperatures.

A use according to the invention in electronic components comprises thefabrication of organic transistors. Specific examples include fieldeffect transitors (FET) having MOS and MIS structures (MOS: metal oxidesemiconductor; MIS: metal insulator semiconductor).

Moreover, the invention can be used in all those electronic componentsin which the very high charge carrier mobility of the highly orderedcolumnar-helical phase can be utilized.

The method according to the invention establishes the columnar-helicalphase, in which the photoconductivity is higher than in the disorderedstate. This can be done in various ways. The orientation can, forexample, be achieved mechanically (by stretching or shearing) or byelectric or magnetic fields. An orientation can also be established bymeans of orienting underlying layers which e.g. include polyimides orare made therefrom. The simplest option is a thermal treatment(tempering).

The present invention has numerous advantages. The low molecular-weightor polymeric organic compounds in the columnar-helical phase exhibit thehigh charge carrier mobilities of organic single crystals (e.g. ofanthracene). Because these organic compounds are easier to orientatethan organic single crystals, their preparation is considerably simpler,however.

EXAMPLES

Example 1 is an example for photoconductors according to the inventionand the method according to the invention for enhancing thephotoconductivity. Comparative Example 1 describes a knownphotoconductor. Example 2 relates to the use in organic transistors.

In Example 1, the organic-photoconductor was studied as follows.

The organic photoconductors were sandwiched between twoelectro-conductively coated transparent glass plates of a glassmeasuring cell.

The spacing of the glass plates was adjusted between 5 and 15 μm bymeans of spacers. Via the conductive layer a voltage was applied to thesample and the current was measured.

For measuring purposes, the glass measuring cell containing the samplelayer to be studied was located in a microscope heating stage, whosetemperature could be regulated, via a heating control system, from roomtemperature to 300° C. at constant heating rates. In a directionperpendicular to the sample surface, the cell was irradiated (intensityabout 0.02 watt/cm²) by a halogen incandescent lamp through a window inthe heating stage cover having a diameter of about 5 mm. The incidentlight beam was modulated by a chopper with a frequency of 10 Hz, i.e.cut up into light pulses having a duration of 50 msec with dark phasesof equal duration.

Via its two contact electrodes (transparent ITO electrodes) themeasuring cell was connected in series with a picoameter to avariable-voltage source. This was used to apply 10 V of DC voltage tothe measuring cell. The picoameter measured the electric current thusgenerated through the sample, i.e. at the light modulation frequencyalternately measured the dark current during the dark phase of theillumination, and the sum of dark current and photocurrent during thelight phase.

The analog output signal of the picoameter was fed to a lock-inamplifier whose reference frequency was provided by the chopper. Here,that fraction of the voltage of the picoameter output was measured whichchanged with the modulation frequency. The voltage fraction was directlyproportional to the difference of the measured cell current in the lightphase and dark phase, respectively, of the exposure to light and wasthus proportional to the photocurrent.

Thus to measure the photocurrent as a function of temperature, themeasuring cell was heated in the heating stage at a heating rate of 5°C./min until the isotropic state was reached. During this temperaturecycle the above-described measuring electronics were active. Thereadings of the lock-in amplifier could be read off at the instrument asa function of the temperature and be converted into units ofphotocurrent.

The experimental values found are shown in the tables.

EXAMPLE 1

2,3,6,7,10,11-Hexahexylthiotriphenylene (H6ST) which had been purifiedby repeated recrystallization was sandwiched in the melt between the twoelectroconductively treated transparent glass plates of the glassmeasuring cell (layer thickness: 32.1 μM), and the photocurrent wasmeasured as described during cooling down to 314K. The experimentalvalues found for the charge carrier mobility in the columnar-helicalphase (H) are shown in Table 1.

COMPARATIVE EXAMPLE 1

Table 2 describes the experimental values for2,3,6,7,10,11-hexapentyloxytriphenylene (HPT) in the diskotic-columnarliquid-crystalline phase (D_(ho)). In this case, the charge carriermobility during heating to 383K was measured (layer thickness: 1.21 μm).

The maximum charge carrier mobilities of the two substances differ inthat the H6ST has a charge carrier mobility which is 2 orders ofmagnitude higher than that of HPT.

                  TABLE 1                                                         ______________________________________                                        (E = 2.0. · 10.sup.4 V/cm, d  32.1 μm, λ = nm)             Phase          T (K)  μ (cm.sup.2 /V.sub.S)                                ______________________________________                                        I              372    9.9 · 10.sup.-5                                               367    9.6 · 10.sup.-5                                               365    9.4 · 10.sup.-5                                D.sub.h        363    1.1 · 10.sup.-5                                               358    1.4 · 10.sup.-3                                               352    1.9 · 10.sup.-3                                               349    2.5 · 10.sup.-3                                               346    3.4 · 10.sup.-3                                               342    4.8 · 10.sup.-3                                H              340    7.2 · 10.sup.-2                                               338    7.7 · 10.sup.-2                                               334    7.6 · 10.sup.-2                                ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        (E 4.0 · 10.sup.4 V/cm, d = 12.1 μm, λ = 337 nm)           Phase          T (K)  μ (cm.sup.2 /V.sub.S)                                ______________________________________                                        D.sub.h        344    7.6 · 10.sup.-4                                               350    7.5 · 10.sup.-4                                               356    7.6 · 10.sup.-4                                               362    7.4 · 10.sup.-4                                               369    7.7 · 10.sup.-4                                               375    7.1 · 10.sup.-4                                               383    6.1 · 10.sup.-4                                ______________________________________                                    

EXAMPLE 2

Randomly substituted hexyl- and pentylthiotriphenylene

A mixture of 4.04 g (0.342 mol) of 1-hexylmercaptan and 3.56 g (0.342mol) of 1-pentylmercaptan is heated with 8.4 g (0.688 mol) of potassiumt-butylate in 50 ml of N-methylpyrrolidone to 70° C. for 10 minutes.After the addition of 4.0 g (0.0057 mol) of 2,3,6,7,10,11-hexabromotriphenylene, the reaction mixture is stirred under nitrogen at70° C. for 30 minutes, then cooled to room temperature and poured intowater. The precipitate is filtered off with suction, washed withmethanol and purified by means of column chromatography. The reactionproduct was characterized by means of ¹ H-NMR spectroscopy (the positionof the signals is identical to those of hexahexylthiotriphenylene, butthe integrals which define the ratio of the alkyl chain lengths aredifferent).

Yield: 4.8 g (95%) Phase behavior: crystalline, at 53° C. helicaldiskotic-columnar phase, at 83° C. diskotic-columnar phase (D_(ho)), at89° C. isotropic. ##STR3##

EXAMPLE 3

Transistor having a MOSFET structure

On top of a silicon wafer having a thickness of 300 μm, an n-typesilicon layer having a thickness of 400 nm was applied as a gateelectrode. On top of this, a silicon dioxide layer having a thickness of200 nm was applied as an insulator. On top of this, a liquid-crystallinelayer of 2,3,6,7,10,11-hexahexylthiotriphenylene was then applied in athickness of from 5 to 10 monolayers by the Langmuir-Blodgett technique.Thus very good orientation of the columns, which are present in theliquid crystal layer, could be achieved in a source-drain direction.Then the source and drain electrodes (each made of gold) werevapor-deposited onto the liquid crystal layer. The width of the channelwas from 0.05 to 0.1 mm, and the length of the channel from 10 to 50 mm.The gate contact consisted of a Ga-In alloy. The analysis of thecharacteristic curves demonstrated that the liquid crystal layer of2,3,6,7,10,11-hexahexylthiotriphenylene was suitable for use intransistors.

We claim:
 1. A method of inducing photoconduction in a photoconductorcomprising the steps of:(a) providing the photoconductor, wherein thephotoconductor comprises low-molecular-weight organic compounds orpolymeric organic compounds, wherein the low-molecular-weight organiccompounds or the polymeric organic compounds have liquid-crystallineproperties and are in the columnar-helical phase; and (b) exposing thephotoconductor to light, thereby inducing photoconduction in thephotoconductor.
 2. The method of claim 1, wherein thelow-molecular-weight organic compounds or the polymeric organiccompounds have a photoconductivity of greater than 10⁻² cm² /Vs.
 3. Themethod of claim 1, wherein the low-molecular-weight organic compounds orthe polymeric organic compounds are triphenylene derivatives.
 4. Themethod of claim 5, wherein the triphenylene derivatives containthioalkyl substituents.
 5. The method of claim 4, wherein thetriphenylene derivatives are 2,3,6,7,10,11-hexahexylthiotriphenylene. 6.A photoconductive layer comprising low-molecular-weight organiccompounds or polymeric organic compounds,wherein thelow-molecular-weight organic compounds or the polymeric organiccompounds have liquid-crystalline properties and are in thecolumnar-helical phase.
 7. The layer of claim 6, wherein the layer has athickness of between 2 and 100 μm.
 8. An electrophotographic recordingmaterial comprising an electroconductive base and a photoconductivelayer,wherein the photoconductive layer comprises low-molecular-weightorganic compounds or polymeric organic compounds, wherein thelow-molecular-weight organic compounds or the polymeric organiccompounds have liquid-crystalline properties and are in thecolumnar-helical phase.
 9. The electrophotographic recording material ofclaim 8, further comprising a sensitizer layer capable of generating acharge carrier.
 10. A method for enhancing the charge-carrier mobilityin low-molecular-weight organic compounds or polymeric organiccompounds,wherein the low-molecular-weight organic compounds or thepolymeric organic compounds have liquid crystalline properties and arecapable of being in the columnar-helical phase, wherein the methodcomprises the step of causing the low-molecular-weight organic compoundsor the polymeric organic compounds to be in the columnar-helical phase,thereby enhancing the charge-carrier mobility in thelow-molecular-weight organic compounds or the polymeric organiccompounds.